In an effort to improve road safety by detecting inattentiveness or fatigue of a vehicle driver, methods are implemented for determining a vigilance state, i.e., an alertness state, of the vehicle driver. This method endeavors to improve road safety by detecting unusual behavior of the vehicle driver and making it known. The unusual behavior may be due to inattention or fatigue, for instance. Fatigue frequently affects the reaction times of the vehicle driver such that reflexes or reactions to particular driving situations take place more slowly, with a delay, and/or less assuredly.
European Published Patent Application No. 1 257 202 describes a method for transmitting a pulse to a vehicle during travel. The pulse is initiated by one or more trigger(s), and the driver usually responds to the pulse spontaneously and subconsciously. The pulse is transmitted to the steering assembly (or to another component with which the driver is normally in contact). Furthermore, a response of the driver is detected by one or multiple sensor(s), such as a steering angle sensor, torque sensor, acceleration force sensor or eye movement sensor. In addition, European Published Patent Application No. 1 257 202 describes that the pulse and the response are compared in order to infer presence of mind of the driver, the presence of mind being inversely proportional to the difference between the pulse and the response. In this regard European Published Patent Application No. 1 257 202 states that the presence of mind of the vehicle driver is detectable in two different ways. In a first alternative, a difference between the pulse and the response is added up and integrated over a specific period of time. The smaller the result of the integration, the better the vehicle driver is able to respond to interruptions. In a second alternative, an amplitude of a centrifugal force in a lateral direction is analyzed. In addition, a reaction time of the driver is able to be determined in the form of a time difference between two different instants, a first instant T1 resulting when the pulse exceeds a predefined threshold value, and a second instant resulting when the reaction exceeds a second predefined threshold value. This has the disadvantage that the threshold-based detection of the first and second instants for determining the reaction time is susceptible to noise. Measuring noise, as it is generated by the afore-mentioned sensors, for instance, may falsify the detection of a response time of the vehicle driver.